The invention relates to a method for treating a liquid, wherein a feed stream is separated into a permeate stream and a retentate stream by means of a membrane arrangement and at least a portion of the retentate stream is discharged from the membrane arrangement as a defined positive displacement volume by means of a positive displacement arrangement.
The liquid can be a solution in which substances, e.g. salts, have been dissolved or finely distributed, for example. Treatment of liquids is necessary for the production of numerous products. Membrane separation methods are especially advantageous since they do not require heating and are generally more favorable in terms of energy than thermal separation methods. For this purpose, a membrane module is fed with a feed stream, which is divided into a retentate stream and a permeate stream. In the separation process, the retentate stream is retained by the membrane. The permeate stream passes through the membrane.
Separation by means of membrane methods has become established in food technology, biotechnology and pharmacy, among others. Depending on the type of membranes used, selective separation of individual substances or certain mixtures of substances is possible.
Among membrane separation methods, distinctions are made according to the driving force behind separation. The present invention relates to pressure-driven processes. An arrangement passes the feed stream to a membrane module. In this case, the arrangement builds up a pressure in front of a semipermeable membrane. The size of the substances retained can be set by means of the choice of membrane. Depending on the size of the molecules retained, a distinction is drawn between microfiltration, ultrafiltration, nanofiltration and reverse osmosis.
The method according to the invention and the system according to the invention prove particularly advantageous in carrying out reverse osmosis, especially in seawater desalination. The liquid is subjected to a pressure which is above the osmotic pressure, and water molecules diffuse through the membrane, while the dissolved salts are retained. Thus, the salt solution, which is referred to as the retentate, is concentrated on one side while desalinated drinking water, which is referred to as the permeate, is obtained on the other side.
The invention relates to a system for treating a liquid in which a positive displacement arrangement is used for energy recovery. In such positive displacement systems, the liquid is delivered by self-contained volumes. In this case, pressure is transferred from the retentate stream to the feed stream. Such positive displacement arrangements are used in small to medium-sized seawater desalination systems since they ensure high pressures, even at small volume flows.
In these positive displacement systems, the permeate stream obtained is dependent on the speed and positive displacement volume per revolution. In these systems, control of the yield is possible only by means of a bypass or drain valve in the high-pressure part. Such control leads to an increase in specific energy consumption.
DE 10 2011 005 964 A1 describes a method for treating a liquid in which a pump passes a feed stream to a membrane unit. In the membrane unit, the feed stream is separated into a permeate stream and a retentate stream. From process data, an evaluation unit calculates the optimum yield at which the system can be operated with a minimum specific energy requirement.
A reverse osmosis system is described in EP 1 986 766 B 1. A positive displacement pump delivers the feed stream to a membrane unit. After the membrane unit, the retentate flows through a pressure recovery unit. The pressure recovery unit is connected to a motor which, in turn, is connected to the positive displacement pump. The pressure upstream of the inlet of the feed stream into the membrane unit and downstream of the outlet of the retentate stream is measured by means of sensors. The speed of the motor connected to the positive displacement pump and to the pressure recovery unit is controlled in accordance with the difference between these pressures. Variation of the yield is possible only by means of a bypass or drain valve. This leads to energy losses.
It is the object of the invention to indicate a method for treating a liquid which, while using a positive displacement system, ensures flexible and energy-efficient operation. At the same time, it should be possible to set an optimum operating point with energy and volume losses which are as small as possible. The method according to the invention is intended to allow different modes of operation, e.g. cost-efficient, energy-saving or membrane-preserving operation. This involves efficient use of the membrane, energy and water pretreatment, e.g. by means of chemicals.
According to the invention, this object is achieved by virtue of the fact that the feed stream is divided into a portion that is fed to the membrane arrangement as a defined positive displacement volume by means of a positive displacement arrangement and into a portion that is fed to the membrane arrangement by means of a delivery unit, wherein the yield is varied by changing the ratio of the speed of the delivery unit to the speed of the positive displacement arrangement. The speeds of the shafts can be varied independently of one another.
In the system according to the invention, both the positive displacement pump and the delivery unit have a dedicated shaft for driving. Thus, in the system according to the invention, two rotary machines are used in parallel to supply the feed stream. The delivery unit is preferably a high-pressure pump, preferably a centrifugal pump. The positive displacement arrangement is a positive displacement pump, preferably an axial piston pump.
Different modes of operation of the system also allow different production rates of treated liquid. While keeping the membrane area the same, the permeate stream can be increased by increasing the pressure in the system. For this purpose, a variable-speed positive displacement or centrifugal pump is used as a delivery unit, for example, which can supply an increasing volume flow as the pressure in the system rises. Similarly, the permeate stream can be reduced by reducing the speed of the delivery unit.
An energy-efficient mode of operation of the system can allow for the possibility of using power from solar or wind energy, for example, when available.
The delivery unit is arranged in parallel with the positive displacement pump, allowing the feed stream to be divided into a portion which flows through the positive displacement pump and a portion which flows through the delivery unit.
At least one shaft is connected to a device for changing the speed. The device is preferably an electric motor that has a frequency converter. The system has a unit which is designed for speed variation. This can be an evaluation and/or open-loop and/or closed-loop control unit.
The arrangement of a variable-speed delivery unit in parallel with a positive displacement pump enables the ratio of the two partial streams to be selectively split. With the aid of process data detected by sensors, the actual state of the system can be determined and adjusted to the optimum operating point by means of the unit.
It is thereby possible to set an optimum operating point with low energy and volume losses. The method according to the invention for operating the liquid treatment system makes possible different modes of operation, e.g. an energy-saving mode or a membrane-preserving mode.
In an advantageous embodiment of the invention, the yield of the liquid treatment system is varied by changing the ratio of the speed of the positive displacement pump to the speed of the delivery unit connected in parallel therewith. By way of the ratio of the speed of the delivery unit to the speed of the positive displacement arrangement connected in parallel therewith, it is possible to set an optimum yield without relatively large energy losses.
Once the optimum yield has been achieved, the desired discharge rate can be set. In this process, the ratio of the speeds between the positive displacement arrangement and the delivery unit connected in parallel is held constant, and the overall speed is either increased or reduced.
In a particularly advantageous embodiment of the invention, the positive displacement arrangement for supplying the feed stream and the positive displacement arrangement for energy recovery from the retentate stream are operated at the same speed. Both positive displacement arrangements are preferably driven by a common shaft arrangement. The shaft arrangement is connected to an electric motor, the speed of which can be changed.
In an advantageous variant of the invention, at least one positive displacement arrangement has a drum with cylinders, in which pistons are arranged. This can be an axial piston pump and/or an axial piston motor. In this embodiment, the positive displacement arrangement comprises a swashplate. The pistons are connected to sliding shoes and have a surface that faces a swashplate. The swashplates are fixed and ensure that the pistons assume different positions during a rotary motion of the drums. In a particularly advantageous embodiment of the invention, the positive displacement volume fed to the membrane arrangement corresponds to the positive displacement volume discharged from the membrane arrangement.
It proves advantageous if both positive displacement arrangements are arranged in a modular unit. A distributor block is preferably positioned between the positive displacement arrangements. Here, the distributor block forms the central component which distributes the liquid streams. For this purpose, the distributor block preferably has a feed stream inlet and outlet and a retentate stream inlet and outlet. In this design, the electric motor which drives the positive displacement arrangements is arranged outside the modular unit consisting of the two positive displacement arrangements and the distributor block positioned centrally between them.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.